According to a dry-wet spinning method, a polymer solution (that is, stock solution for spinning) is firstly discharged from a spinning hole of a spinneret to a vapor phase part (generally, in air) for forming fibers, which are then coagulated by introduction to a coagulation bath, and subsequently the fibers are taken up from the coagulation bath to form a fiber bundle. According to the dry-wet spinning method, since the fiber draft generated by taking up of fibers is concentrated on a vapor phase part, it is possible to achieve coagulation·gellation of fiber under low tension in a coagulation bath. Accordingly, a fiber bundle having an excellent extension property at post process can be obtained. According to the dry-wet spinning method, a fiber bundle consisting of staple fiber having excellent densification can be obtained.
Meanwhile, there is a need for reducing the cost for manufacturing a carbon fiber bundle. As one method to meet such need, there is an improvement of productivity of an acrylic fiber bundle that is required for manufacturing a carbon fiber bundle. For the improvement of productivity, it would be necessary to have high speed spinning and high density spinning (that is, having many spinning holes on a spinneret) of an acrylic fiber bundle.
However, in case of high speed spinning, as the running speed of a fiber bundle passing through a coagulation bath is increased, the flow amount of coagulating liquid which flows accompanying the travel of a fiber bundle is increased. According to an increased accompanying flow, the flow amount of a coagulating liquid is further increased in the coagulation bath, and as a result, the liquid surface of a coagulating liquid is swollen and sometimes a phenomenon of having whirlpool occurs. When such phenomenon occurs, liquid surface vibration of the coagulating liquid that is present right below the spinneret is increased. The liquid surface vibration of a coagulating liquid causes a disturbance in arrangement of staple fibers or yarn breakage of staple fibers in a fiber bundle. When the liquid surface vibration of a coagulating liquid is significant, part or all of the surface (spinneret surface) on which spinning holes of a spinneret are arranged may be in contact with the coagulating liquid, making it impossible to carry out dry-wet spinning.
For high density spinning, that is, in case of having many spinning holes on a spinneret, when distance between adjacent spinning holes is reduced to have many holes and liquid surface vibration of the coagulating liquid is high, a phenomenon of having neighboring staple fibers adhered to each other is caused while a fiber spun from the spinning hole firstly passes through a vapor phase part, that is, before the fiber is coagulated.
To solve such problems, for example, in JP 1-183511 A (Patent Document 1), a device equipped with a straightening pipe for surrounding running yarn is disclosed. It is also described that the top surface of the straightening pipe needs to be exposed above the liquid surface. In JP 7-207522 A (Patent Document 2), plural pieces of straightening plates or a waviness preventing plate composed of a porous plate are disposed vertically between a spinneret and a coagulation bath wall, which is opposite to the take-up direction of spun yarn, or between a spinneret and a guide on take-up side of a coagulation bath. In that case, similar to Patent Document 1, the top part of a straightening plate or a waviness preventing plate is also exposed above the liquid surface.
Further, in JP 11-350245 A (Patent Document 3), a method of controlling waviness on liquid surface of coagulation bath by floating a ball on the liquid surface of coagulation bath in an area below the outer periphery of a spinneret is suggested. Further, in JP 2007-291594 A (Patent Document 4), a method of installing a partition plate at a distance from a yarn within a range of 20 to 200 nm in a coagulation bath which follows a yarn during the passage of downward running in the coagulation bath via a funnel-shape straightening plate, turning back with the direction changing guide, and then guided to a guide on take-up side is disclosed.